What could this be???

Well I was on my way to work when I was jamming AudioSlave on 93.5 fm... well I to a 4 way inersection and I herd this odd tone I never herd before, it rang about 2 seconds long with a 1 second pause and the tone was low then got higher... it was odd and it happened 3 times before it stoped (I went about 30 feet or so)
it was in the middle of traffic lights what i find odd... has anyone herd this before???